1. Field of the Invention
The present invention relates to a light-emitting diode (LED) for use of display or the like.
2. Description of the Prior Art
In recent years, there have been developed light-emitting diodes (LEDs) that emit yellow or green light using aluminium gallium indium phosphide (AlGaInP) series materials, besides those using gallium arsenide phosphide (GaAsP) series or gallium phosphide (GaP) series materials.
A prior-art AlGaInP series LED is fabricated in the following way. First, as shown in FIG. 1 which is a sectional view of the LED, on a surface of an n-type GaAs substrate 90 are stacked over all an n-type AlGaInP clad layer 91, an undoped AlGaInP light-emitting layer 92, a p-type AlGaInP clad layer 93, a p-type AlGaAs current diffusion layer 94, a p-type GaAs contact layer 95, and a p-side surface electrode 96. Next, as shown in FIG. 2 which is a plan view showing the LED, this surface electrode 96 and the p-type GaAs layer 95 are patterned by removing part of them. As a result, the surface electrode 96 is composed of a circular pad 98 for performing wire-bonding, and branches 99a, 99b, 99c, and 99d linearly extending from the pad 98 in four directions. By providing a plurality of branches 99a, 99b, 99c, and 99d in this way, it is arranged that electrical current is diffused within a LED chip as uniformly as possible. Thereafter, an n-side rear-face electrode 97 is formed on the rear face of the substrate 90. Light emitted from the light-emitting layer 92, being absorbed into the substrate 90 and the surface electrode 96, goes out of the chip through a region 100a of a chip surface 100 resulting from partially removing the surface electrode 96 and through a side face 101.
It should be noted that this LED is of double-hetero structure in which the light-emitting layer 92 is sandwiched by the two clad layers 91 and 93 greater in bandgap than the light-emitting layer 92. With this structure, to attain an effective confinement of electrons and holes into the light-emitting layer 92 by the clad layers 91 and 93, the Al composition ratio y is required to be increased to as much as 0.7 to 1 in the composition (Al.sub.y Ga.sub.1-y).sub.0.5 In.sub.0.5 P of the clad layers 91 and 93. However, if the Al composition ratio y is increased to such a value, doping a p-type or n-type element into the clad layers 91 and 93 becomes difficult so that the specific resistance of the clad layers 91 and 93 becomes difficult to lower. Therefore, in this LED, the current diffusion layer 94 is provided to prevent the current from concentrating underneath the surface electrode 96, thereby increasing the amount of light emitted in the region 100a that is not covered with the surface electrode 96.
However, the current diffusion layer 94 does not work to a sufficient extent, so that ineffective light emission underneath the surface electrode 96 is greater in amount relative to light emission in the region 100a that is not covered with the surface electrode 96. This accounts for the fact that the prior-art LED is worse in external quantum efficiency, as one problem.
Another problem is that since the wavelength of light emission is in the range of 590 nm (yellow) to 550 nm (green), there will occur light absorption in the AlGaAs current diffusion layer 94. This is attributed to the fact that even if Al.sub.x Ga.sub.1-x As was set to a composition ratio of x=1 that gives the widest bandgap, its absorption end would be 574 nm, not allowing the light of shorter wavelengths to pass through the layer. In addition, AlAs (corresponding to x=1) is susceptible to corrosion in air, unsuitable for use as the surface layer.
Another example of the prior-art AlGaInP series LEDs is such as shown in FIGS. 3 and 4. FIG. 3 shows the surface of the LED and FIG. 4 is a sectional view taken along the line IV--IV as indicated by arrows in FIG. 3. This LED is fabricated in the following way. First, as shown in FIG. 4, on a surface 180 of an n-type GaAs substrate 190 are stacked over all an n-type AlGaInP clad layer 191, an undoped AlGaInP light-emitting layer 192, a p-type AlGaInP clad layer 193, a p-type GaAs contact layer 194, and a surface electrode (e.g. AuZn) 195. Next, the stacked surface electrode 195 and the layers 194, 193, 192, and 191 are selectively removed until a substrate surface 190a is reached, leaving specified portions to provide mesa (trapezoidal) portions 200. The pattern of the mesa portions 200 (approximately identical to the pattern of the surface electrode 195) is, as shown in FIG. 3, provided in combination of a pad 198 for performing wire-bonding, lateral branches 199a and 199b extending from the pad 198, and longitudinal branches 200a, . . . , 200h crossing the lateral branches 199a and 199b. Thereafter, as shown in FIG. 4, a rear-face electrode 196 is formed on the rear face of the substrate 190.
Referring to the longitudinal branch 200h in FIG. 4 by way of example, light emitted from the light-emitting layer 192 goes out of the LED primarily through right and left side faces 201a and 201b of a mesa portion 200. This is attributed to the fact that the light beams traveling upward and downward will be absorbed into the surface electrode 195 and the substrate 190. As will be understood from this, the reason the mesa portions 200 are formed on the substrate surface 180 is to take light out of the LED efficiently by increasing the area of a light outgoing face (mesa slant face).
However, the construction of the mesa portions 200 is no more than such an arrangement that the lateral branches 199a and 199b and the longitudinal branches 200a, . . . , 200h merely cross each other, involving relatively long longitudinal lengths of the branches. On this account, most of the light emitted in the longitudinal direction (back and forth in FIG. 4) within each of the branches 200a, . . . , 200h will not reach the ends (e.g. an end 201c) of the branches, resulting in ineffective light emission. As a result, the above prior-art LED is not so good in external quantum efficiency, disadvantageously.